Katy Perry is about to get slammed for doing what celebrities have done for decades: shill soft drinks.

On Tuesday, a handful of consumer advocacy groups, including the influential Center for Science in the Public Interest, will run a full-page ad in Variety, the Hollywood trade publication, asking the superstar singer to stop marketing Pepsi to young folks.

"It's a sad story that some of the best-known celebrities in the country are encouraging their young audiences to drink beverages that are bad for their health," says Michael Jacobson, executive director of the group. "We're focusing on Katy because she's so popular with young people."

The goal: to ultimately make celebs gun-shy about taking on lucrative soft drink endorsement deals. Perry, who is releasing a new album, Prism, is the first celeb in the public sights of the advocacy group. There will be more, says Jacobson.

At stake: hundreds of millions of endorsement dollars that the cola giants have put in the hands of America's most famous celebrities including Michael Jackson, Cindy Crawford and Beyoncé.

The ad says: "Being popular among children brings with it an enormous responsibility. Don't exploit that popularity by marketing a product that causes disease in your fans."

One in three children is overweight or obese, says Jacobson. And consuming one additional sugary drink a day raises a child's risk of becoming obese by 60%, he says.

But one celebrity endorsement expert doubts the campaign will have much effect. "It may depend on how big this blows up," says Noreen Jenney Laffey, president of Celebrity Endorsement Network. "Ultimately, I'd be surprised if people turn down these multimillion dollar deals."

Pepsi has no long-term endorsement deal with Perry and has never technically had her under contract, but worked through third parties. At the recent Video Music Awards, Pepsi asked fans to vote on which song Perry performed. And Pepsi and Perry also linked up on a campaign for her concert film, Part of Me 3D.

Perry's representatives declined to comment. But Pepsi spokeswoman Andrea Foote said, in a statement, "We have a long history of responsible advertising and marketing practices, including a commitment to not direct our advertising to audiences comprised predominantly of children under 12."

Jacobson says his group focused on Perry because of her popularity with kids. She has 58 million Facebook fans, many of them youngsters. She has 45 million Twitter followers, about half under age 16, reports Demographics Pro, a social media market research firm.

These kids all receive the same message from Perry's Pepsi ads, says Jacobson: "Live for now — and worry about the health consequences later."